1. Field of the Invention
The present invention relates to a navigation system using an angular rate sensor, and more specifically to a navigation system by which automotive vehicle travel locations are detected by an angular rate sensor and a distance sensor and the detected vehicle locations are projected on a displayed road map so that the vehicle travel display moves along a predetermined route on the map.
Since this system can be operative independently without need of other auxiliary means (e.g. radio signals), this system is effective in an urban area or other areas subjected to radio interference where radio navigation systems cannot operate reliably.
2. Description of the Prior Art
The applicant has already proposed a dead reckoning and map correlation system for automotive vehicle tracking, which uses an angular rate sensor and a distance sensor, in Japanese Unexamined Published (Kokai) Patent Application No. 60-48600, entitled vehicle position detecting system. In this system, current vehicle locations can be intermittently detected on the basis of vehicle travel distance data and vehicle travel angle data (detected by an angular rate (velocity) sensor); and the calculated vehicle locations are projected onto roads of a digital map (including intersections and inflections of roads) previously prepared by inputting digital map data to a CPU through a keyboard so that the vehicle travel motion can track a road displayed on the map.
In the prior-art navigation system, a route R is calculated on the basis of map information data including intersections, inflections, and point numbers adjacent to succeeding points all stored in a memory unit, and the current vehicle travel location is displayed along the calculated route R. Further, as shown in FIG. 1, a route judge area JE is formed at each branch point P.sub.1 in order to determine a direction along which the vehicle travels. That is to say, a vehicle travel angle is integrated within this route judge area, and the integrated vehicle travel angle is compared with the angles .theta..sub.2 and .theta..sub.3 of the routes P.sub.2 and P.sub.3 with respect to a horizontal line X.
Therefore, in the prior-art navigation system, although the vehicle travel locations can be displayed along the route R stored in the memory unit, when the vehicle turns unexpectedly into a gasoline station, for instance, since the vehicle location is still displayed only on the route R obtained by the map, there exists a problem in that the unexpected vehicle travel is displayed as if the vehicle travel in a straight line.
In addition, when the vehicle travel route change is not correctly detected at a branch point, there exists a problem in that vehicle travel locations are not correctly displayed along the route R, and there exists a deviation from an expected route on the map.